The movement of load carrying vehicles through a conduit by air in which the vehicles are suitably collected, stored, and dispatched are disclosed in copending patent application Ser. No. 894,108 entitled "Method and Apparatus for Processing Vehicles Moving Through a Conduit by Air" filed Apr. 6, 1978, now U.S. Pat. No. 4,165,845. In that system, the pump was positioned with its suction intake connected to the downstream side of the processing system. It is desirable in some applications to place the air pump in parallel with the processing system thereby utilizing the pump to move vehicles downstream of the processing system by a positive pressure and at the same time draw vehicles to the processing system by the pump suction. However, the application of the pump suction to the upstream side of the processing system tends to cause reverse air flow through the processing system and undesirably acts on the vehicles in a direction which tends to move them upstream.
The present invention is directed to various improvements in an apparatus for controlling vehicles moving through a conduit by air in which a pump can be positioned in parallel with the processing system and suitable valve means connected to the conduit for reducing the tendency of the vehicles in the processing system to move upstream.